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Shabbos 28b says that the bull that Adam Harishon brought as a sacrifice had one horn.
One is the kapores of the aron.
The Kapores is unique. There are multiple krashim, adanim, and badim, the paroches had a pair, and every other kli was duplicated multiple times by Shlomo HaMelech except the aron (Moshav Zekenim Parshas Terumah 25:21). But the structure of the aron itself is similar to that of the mizbach hanechoshes - a wooden box covered with metal. The only differences are the size and orientation of the box!
Only the kapores and the menorah were hammered directly out of gold, but the Shlomo had 10 duplicates of the menorah made, but he could not do so for the aron and the Kapores.
So the only truly unique piece is the kapores. Why?
Because the two angels facing each other represent and reflect the relationship between Klal Yisroel and Hashem. (See Yuma 54a)
We have two sets of luchos - the whole and the broken. There is the tzintenes hamon that testifies to the chessed of Hashem, but Chazal say that the mon melted and was drunk by the animals so that the umos ha'olam could experience it. All of these fill the inside of the aron because they form the foundation for the kapores: the representation of the one truly unique thing in the briyah - Hashem's loving relationship with US!